This patch provides improved infrastructure for developing test262 tests
together with V8. It has three parts:
- The test262 test runner is updated to look for local versions of tests
  in the /test/test262/local-tests directory, which mirrors
  /test/test262/data. Additional tests can be added there and are run
  together with tests from upstream. Upstream tests can be locally
  updated by using the same name in local-tests; if a same-named test
  exists, then only the local version will be run. The local-tests
  directory is in the V8 repository, unlike the contents of the data
  directory, so tests can be added in the same patch as something else.
- The tool /test/test262/upstream-local-tests.sh is added to create
  a patch against the test262 respository based on a patch which changes
  the local-tests directory.
- The tool /test/test262/prune-local-tests.sh is added to remove
  redundant local tests on a test262 roll.

See design doc:
https://docs.google.com/document/d/16bj7AIDgZLv4WOsUEzQ5NzcEN9_xo095e88Pz8FC5rA/edit

Review-Url: https://codereview.chromium.org/2611793002
Cr-Commit-Position: refs/heads/master@{#42117}